Been using amsoil for years in all my rides,but recently picked up a Terra and the recommended oil for it is Castrol 4t racing oil.
It looks similar to the motor ex in the fact the oil is almost colorless,which I like because you can look at it and actually tell how dirty it is....if that even means anything...
...and I can get in 10w50
At the asking price it ought to be good.Just like to find a cheaper supplier.The local dealer gets the same price as the internet guys..
castrol